A synchronous optical network (SONET) is well known in the art and includes network elements (NEs) such as Add-Drop Multiplexers (ADM's). With respect to operations data networking, the SONET NEs are either Gateway NEs, Intermediate NEs, or End NEs as illustrated in FIG. 1.
SONET signals have rates and formats as defined in BELLCORE TECHNICAL REFERENCE entitled "Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria", TR-NWT-00253 (TR 253), which contains many criteria that are applicable to SONET NEs. This is realized by defining a basic signal of 51.840 Mb/s and a byte interleaved multiplex scheme. The basic signal can be divided into a portion assigned to overhead and a portion that carries the payload. Payload pointers are mechanisms that allow the payload to shift relative to the overhead, thus permitting the accommodation of different signal phases and frame rates in multiplexing.
The Synchronous Transport Signal-level 1 (STS1) is the basic modular signal. Its rate is 51.840 Mb/s. The optical counterpart of the STS-1 is the Optical Carrier-level 1 signal (OC-1), which is the result of a direct optical conversion of the STS-1 h after frame synchronous scrambling.
The definitions of the first levels (STS-1 and OC-1) define the entire hierarchy of synchronous optical signals because the higher level signals are obtained by synchronously multiplexing lower level signals. The higher level signals are denoted by STS--N and OC--N, where N is an integer. There is an integer multiple relationship between the rates of the basic module OC-1 and the multiplexed signal OC--N (i.e., the rate of OC--N is equal to N times the rate of OC-1).
SONET optical transmission systems support only certain values of N. Currently, these values are 1, 3, 12, 24 and 48. Table I lists standard optical carrier rates from 51.840 Mb/s up through 2488.320 Mb/s.
TABLE I ______________________________________ Line Rates for The Allowable OC-N Signals OC Level Line Rate (Mb/s) ______________________________________ OC-1 51.840 OC-3 155.520 OC-12 622.080 OC-24 1244.160 OC-48 2488.320 ______________________________________
The SONET transport format presented here is based on ANSI T1.105. FIG. 2 depicts the STS-1 synchronous payload envelope (SPE). The STS--N signal is formed by byte innerleaving N STS-1 signals. The Virtual Tributory (VT) structure is designed for transport and switching of sub-STS-1 payloads. There are four sizes of VTx: VT 1.5 (1.728 Mb/s), VT2 (2.304 Mb/s), VT3 (3.456 Mb/s), and VT6 (6.912 Mb/s). In the 9-row structure of the STS-1 SPE, these VTs occupy 3 columns, 4 columns, 6 columns, and 12 columns, respectively.
The overhead and transport functions are broken into layers that increase in complexity from the viewpoints of hardware and the optical interface frame format. The layers are Physical, Section, Line and Path, as illustrated in FIGS. 3 and 4. The layers have a hierarchical relationship and are considered from the top down. The top-down approach provides a general introduction to the individual layers and their functionalities.
Each layer requires the services of all lower level layers to perform its function as illustrated in FIG. 4. For example, suppose that two Path layer processes are exchanging DS3s. The DS3 signal and STS POH are mapped into an STS-1 SPE, which is then given to the Line layer. The Line layer multiplexes several inputs from the Path layer (frame and frequency aligning each one) and adds Line overhead (e.g. overhead required for protection switching). Finally, the Section layer provides framing and scrambling before optical transmission by the Physical layer.